1. Field of the Invention
The present invention relates generally to an apparatus and method for enhancing images, and, more particularly, to an apparatus and method for enhancing images, having the limited dynamic range of a brightness level, such as images taken in a backlight compensation mode, to high dynamic range images.
2. Description of the Related Art
As is well known in the art, dynamic range means difference in brightness between the darkest and brightest discriminative regions of interest in an image. For example, in the case of a black and white image of 8 bits, if brightness is not saturated in both a dark region and a bright region, the dynamic range of this image becomes 256:1. Human eyes can detect a scene with a dynamic range of 10000:1 or more. However, if this scene is taken by a digital camera, all regions cannot be regularly expressed, so that some regions are saturated and the information thereof is lost during a process of quantizing the scene into discrete brightness levels (chiefly, 8 bits for the single color channel of a pixel)
For example, if an image is taken in a backlight compensation mode, a subject is darkened due to underexposure, so that there is a disadvantage in that the region of interest of the subject cannot be detected. If exposure is increased in order to alleviate such a disadvantage, the region of interest of the subject can be detected but the brightness of a region, such as a background, which has not been affected by backlight, is saturated due to overexposure, so that the region of interest of the background cannot be detected.
In order to solve the problems, a plurality of methods has been proposed. There is a hardware method of using an image sensor for obtaining images taken at multiple exposure times with a single image-taking. With regard to other accessing methods of taking images using existing cameras and processing the images as signals, there are methods such as gamma correction and histogram equalization.
In the case of gamma correction, the brightness distribution of a dark region is enlarged, so that the region of interest of the dark region can be detected. However, since a single Look-Up Table (LUT) is used, the brightness distribution of a bright region should be excessively compressed, so that there are problems in that information is lost in the bright region and the saturation of colors is lowered. Similarly, in the case of histogram equalization, since a single LUT is used, there are disadvantages in that the overall contrast is not natural and color image distortion is bad. In addition, although there is a plurality of methods using a single LUT, the excessive compression is inevitably performed on other regions in order to enlarge a dark region, with the result that local contrast cannot be considered, so that there are common disadvantages in that only a intensity of pixels of input image controls the enhanced intensity of pixels and information based on the location of the pixel is not considered.
In order to overcome the above-described limits, methods of using a plurality of LUTs have been proposed. The simple example thereof is an adaptive histogram equalization method of equalizing a histogram for each block. This method has an advantage of considering local contrast. However, since the number of brightness levels within a block is small, there is a disadvantage in that a Halo effect is generated and noise is amplified during a process of equalizing a histogram. In addition, there is a method of applying different LUTs depending on locations. However, in the case in which a subject, the original color of which is dark because reflectance from an object is low, exists in a bright region, the subject cannot be distinguished from a subject which is darkened due to backlight, so that there is a problem of distorting the brightness and color information of a normal object. The reason for this is that, when pixels are enhanced using the above-described methods, even though the pixels have different enhancement characteristics depending on the locations thereof by referring to information about adjacent pixels, the information about adjacent pixels to be referred to is limited due to the amount of operation. It is difficult to accurately distinguish a subject which is darkened due to backlight from a subject which is originally dark due to the unique reflectance of an object. However, in order to obtain enhanced images having better image qualities, it is required to use different enhancement methods even in the same dark region depending on the cause, that is, low reflectance and backlight.